1. Technical Field
This invention relates to cordless telephone systems and more particularly to an arrangement for extending the operating range of a portable unit within a cordless telephone system.
2. Description of the Prior Art
Cordless telephones in the United States presently operate on ten separate frequency pairs (channels) in the 46/49 MHz band. These telephones have generally been used in the residential market to allow the user to make and receive telephone calls with a portable handset unit connected by a radio link to a fixed base unit. Because of radiated power limitations on the radio link, however, the handset unit must be used within a radius of a few hundred feet of the base unit. The actual range of operation in any given situation depends on the radio propagation characteristics of a particular premises, which are affected by the building material composition as well as the contents of the building. Generally, the operating range limitation has not been considered a shortcoming for the cordless telephones that are used in residential premises, but now that cordless telephones are being provided for use in business premises, this operating range limitation is considered a much more critical problem. Moreover, business premises generally provide a harsher electromagnetic environment than residential premises, with the building material composition and building contents being such that they significantly reduce the operating range of the cordless telephone from what it would otherwise be if operated in a residential premises. In addition, the business customer is more critical of communication problems than the residential customer and will view radio deadspots as unacceptable.
In order to cope with the limited operating range of the cordless telephone system, one multiple channel access system employs a dedicated common signaling channel to set up calls. Operation of the system is such that the handset unit bids for access to the plurality of radio frequency channels. Access to channels is provided by a centralized controller through a transceiver at a cell site. Since cordless telephones have been allocated a limited number of communication channels over which they may operate, this system has the disadvantage of reducing the available number of channels by minimally committing one of these as the common signaling channel. This system also has the disadvantage of requiring manual location registration wherein the user of the portable handset unit must remember to register the location of his or her handset unit, else the centralized controller will not known the whereabouts of this portable handset unit. One alternative to manual location registration is the suggestion of periodic polling of each portable handset unit through the transceivers at the cell sites. This is also undesirable, however, in that it may cause needless interference to users of other nearby cordless telephone systems.
In addressing the problems of limited available radio frequency spectrum and the resulting interference between transceivers operating at the same frequencies, one cordless telephone system employs a line-of-sight or optical transmission link for communications between portable handset units and dispersed base units. This system, described in U.S. Pat. No. 4,456,793, specifically utilizes modulated infrared transmissions between the portable handset units and each of the dispersed base units. Although this system generally avoids the problems associated with having limited available radio frequency spectrum, it has the disadvantage of having a limited operating range between base and handset units because of its line-of-sight operating requirement. It unfortunately thus requires a large number of base units dispersed through a premises in order to obtain the desired continuous coverage for a user of a handset unit as he or she roams through the premises.